In general, oil is produced off shore at great depth from a floating support anchored close to oil wells situated on the sea bottom, i.e. at depths that lie in the range 1000 meters (m) to 2500 m, or even more. The floating support generally has anchor means so as to enable it to stay in position in spite of the effects of currents, winds, and swell. It also generally includes storage means and oil treatment means together with means for offloading to tankers, which dock at regular intervals to offload the production. These floating supports are commonly referred to as floating production storage offloading supports, and they are designated by the abbreviation “FPSO” throughout the description below.
In general, well heads are connected to said FPSO by undersea pipes either of the steel catenary riser (SCR) type, i.e. pipes that are suspended in a catenary configuration, or else of the hybrid tower type comprising:                a vertical riser having its bottom end anchored to the sea bottom and connected to a said pipe resting on the sea bottom, and whose top end is tensioned by a float immersed under the surface and to which it is connected; and        a link pipe, generally a flexible link pipe, between the top end of said riser and a floating support on the surface, said flexible link pipe presenting, where appropriate and under its own weight, the shape of a dipping catenary curve, i.e. a curve that goes down well below the level of the float before subsequently rising up to said floating support.        
The entire crude oil production is thus generally raised on board the FPSO where it is treated in order to separate the oil proper from water, gas, and any sandy components. Once separated, the oil is stored on board, the gas is washed and then delivered to gas turbines to produce the electricity and heating required on board, and any surplus is reinjected into the oil field reservoir so as to restore pressure in said reservoir. After being freed of the sand in suspension, the water is finally rejected to the sea after thorough extraction of any particles of oil, or else it too is reinjected into the reservoir, generally together with additional sea water taken from the sub-surface that generally needs to be added in order to achieve the flow rate needed for injecting water into the reservoir. The extracted sand, which constitutes quantities that are very small, is finally washed and rejected into the sea.
When an oil field begins to be worked, water represents only a small percentage of the crude oil, but after several years, or even several tens of years, water produced can represent 80% to 95% of the production, and installations for separating, treating, and reinjecting water need to be dimensioned accordingly. It is thus necessary to raise to the surface very large quantities of water that are subsequently returned to the sea bottom for reinjection into the oil deposit in the reservoir.
Numerous separator systems have been developed over tens of years for separating liquids, gases, and solids, and in particular for separating oil, water, gas, and particles of sand.
The method commonly used in installations on land is to provide tanks of very large volume, generally in the form of elongate cylinders, with crude oil entering at one end and running along said tank over a period of about 5 minutes (min) during which the various phases separate naturally under gravity so as to reach the second end where the gas is recovered from the top of the tank, the water and the sand from the bottom, and the oil from an intermediate portion. Separators of that type exist in very great variety and they generally incorporate additional internal devices such as horizontal, vertical, or sloping screens, for the purpose of facilitating separation of the phases and preventing them remixing at a subsequent stage.
Unlike the above-described horizontal separators that make use of the force of gravity for performing said separation, cyclone type separators make use of centrifugal force and they are often used because they are compact, however they remain very difficult to operate because they present operating points that are quite narrow, and as a result they do not accommodate large variations in the oil/water and liquid/solid ratios (the term “operating point” being used here to designate an operating region in which volumes of phases of different densities within the cyclone in stable manner). The fluid remains in a separator of that type for a very short length of time, of the order of 0.5 seconds (s), and as a result a very sophisticated monitoring and control system needs to be implemented in order to adjust all of the servo-valves so as to ensure that the parameters of the system can be kept stable about an optimum operating point. Passive type cyclone separators exist in which the separation energy is taken directly from the fluid by creating a loss of head, generally of a few bars. Other variants are based on delivering external energy, by making use of a motor, generally an electric motor driving a device that is generally of the bell type, for the purpose of creating said centrifugal force within the fluid to be separated, such as Dynaclean systems from Neyrtec (France).
Patents WO 97/28903, EP 0 259 104, and WO 97/15368 describe cyclone type liquid/liquid separators of the kind that have a tangential inlet for the fluid for separation, leading to a first portion of the cyclone that is cylindrical, which cylindrical portion is extended by a bottom portion that is conical, tapering only slightly, and of the type described below with reference to FIGS. 1 and 1A to 1E, leading to an axial outlet orifice for the heavy phase (water), while the light phase (oil) is discharged by an axial outlet orifice at the top end of said cylindrical first portion.
As explained below, the centrifugal force of the fluid delivered to the top end of the cyclone via a tangential inlet sets up a central first volume of the light phase (oil) that is surrounded by a peripheral volume of the heavy phase (water).
When the incoming fluid flow rate varies, and particularly when the respective proportions of water and oil in the fluid for treatment vary, it is necessary to adjust the operating parameters of the cyclone in particular so as to avoid pockets of oil separating and being entrained with the water.